Gas turbine engines include a compressor that compresses air, a combustor that ignites the compressed air and a turbine across which the compressed air is expanded. The expansion of the combustion products drives the turbine to rotate, which in turn drives rotation of the compressor. In some examples a fan is included forward of the compressor section, and driven to rotate by the turbine as well.
In order to ensure maximized efficiency of the gas turbine engine, a clearance between the tips of rotors in the primary flowpath and an inner diameter of the primary flowpath is kept sufficiently small that a minimum amount of air passes between the tip of the rotor and the outer diameter of the flowpath, while still allowing sufficient space that the tip of the rotor does not contact the outer diameter.